In a private network with non-standard nodes you could choose larger values for the RX1 and RX2 delays*. But then any off-the-shelf node would no longer be supported, as that will still expect 5 and 6 seconds for OTAA. 
How’s the latency when using another network? (Just to ensure it’s really only GPRS that is to blame.)
*) and send the downlink to the gateway as early as possible:
(I think there’s a post somewhere explaining one can ensure the downlink is sent to the gateway in time.)